Don't route mouse wheel events to transparent children (Legacy_RenderWidgetHostHWND).
Don't route mouse wheel events to transparent children (Legacy_RenderWidgetHostHWND).

Mouse wheel events on Windows would get routed to the window at the current location if it
is in the same process. The message is routed via the SendMessage call. Scrolling via the
wheel on a webpage would end up routing the mouse wheel message received for the main window
to the the legacy window which would then callback into the main window. While the SendMessage
call within the same thread works like a function call, processing the whole callstack twice
causes a lot of jank while scrolling.

Fix is to ignore transparent children in the RerouteMouseWheel function.

BUG=462132

Review URL: https://codereview.chromium.org/1261253003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#341047}

ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend